Detector for phase failure in polyphase electrical systems



Nov. 17, 1964 R. L. NORTON 3,157,826

DETECTOR FOR PHASE FAILURE IN POLYPHASB ELECTRICAL SYSTEMS Filed Dec.50, 1959 43 L 43"] as j Robert L A/orfan INVENTOR.

ATTORNEY! United States Patent DETECTQR This invention relates to adevice for detecting a phase failure in polyphase sources of electricpower.

For purposes of the following discussion and claims a phase source" maybe defined as any two conductors having a single phase of a polyphasepower supply impressed upon them and avaiiable to supply current of thatsingle phase to either a single phase load or part of a polyphase load.A polyphase source is defined as more than two such conductors having apolyphase power supply impressed upon them and available to supply suchpolyphase power to either a plurality of single phase loads or to apclyphase load. Any single conductor may of course be common to aplurality of phase sources.

Polyphase electric power sources and in particular the three-phasesources are in extensive use in industry to operate polyphase inductionmotors and other current consuming apparatus. The failure of one of thephase sources results in some of the polyphase apparatus operating undersingle-phase conditions. Operation under single-phase conditions ofelectrical apparatus designed for polyphase conditions will result indamage to them due to the excessive current passing therethroug'n underthe non-normal phase conditions. Additionally, a phase failure resultsin at least a partial reduction of voltage therein. Naturally, operationunder reduced voltage will produce excessive harmful currents insingle-phase or polyphase apparatus operated therefrom.

A device automatically indicating the failure of one or more of thephases in polyphase sources is very desirable. Such devices must beadapted to give satisfactory operation under conditions existing in suchpolyphase sources connected to current consuming loads.

One of the conditions existing in polyphase sources is the generation ofa counter voltage into the phase which has failed by certain apparatusconnected thereto, such as electric induction motors. This countervoltage may be between to 85 percent of the normal voltage in that hase.The device must therefore be capable of distinguishing the countervoltage from the normal operating voltage in the failed source.

Another condition is the variation of voltage in all the sources andbetween one and another of the sources during normal operation. Thesevariations occur when the loading to the polyphase sources changes andthe changes in loading may be caused by connection of large currentconsuming motors to the polyphase sources or to the changes in the workload applied to these motors. Additionally, some variations in voltageoccur in the distribution systems of such polyphase sources. Thesevariations are normal and should not activate the device to indicatephase failure.

The voltages in each phase of a polyphase source may not be identicalunder normal operating conditions. Thus the device should be capable ofbeing adjusted to the actual voltages in each phase, which are normaland not necessarily identical.

There also exists the condition of unbalanced current distributionbetween the polyphase sources. This condition is created where there aresingle-phase loads connected between the polyphase sources in additionto polypha-se loads. The current carried by any one phase source is nota fixed value but varies as the work load "ice of the current consumingapparatus changes and as the number of such apparatus connected to thepolyphase source changes. It is to be noted that under certain normaloperating conditions some of the sources of the polyphase sources maycarry different currents due to the above types of loads used in thissystem. The phase failure detection device should not be activated bysuch current changes or by the normal differences in current carried bythe phase sources during normal operation.

The devices heretofore used have not been found to be satisfactory forone reason or another. These prior devices may be classed according tothe electrical characteristic that is monitored. The classes of thesedevices are voltage sensing, current sensing, and phase sensin hevoltage sensing devices utilized voltage sensitive relays which areadjusted to operate at a preset percentage difference in voltage from agiven voltage. These devices operate when one or more of the phasevoltages fal below that percentage diiference from the given value.These devices do not compare one phase voltage with one or others of theremaining phase voltages, but use the total sum or difference of thevoltages in all sources as the monitored voltage. Upon the presetpercentage difference in this monitored voltage from the given voltage,the voltage sensing relay is activated. For this reason, thefluctuations in the polyphase voltages due to normal variations indistribution or the loading of the polyphase source by the equipment ofa plant can activate these devices. This results in a false indicationof phase failure.

The current sensing devices utilize current balanced transformersconnected between the phase sources and such transformers must beclosely matched to the current carried in each phase source. Naturally,these current sensing devices cannot be used where the total current ina polyphase system normally varies or an unbalancing of the currentsbetween the phases normally occurs. Devices of this type are generallylimited to polyphase sources where the current flow in each source isuniform and there are no changes in currents which unbalance the currentrelationship existing between phases. Connection of a single-phaseapparatus to the polyphase sources would upset such a current balanceand result in a false indication of phase failure by this type ofdevice.

One of the types of phase sensing devices utilizes mechanical cams andtrips driven by normally phase balanced induction motors operated fromthe phase sources. These devices have mechanical parts which becomeinoperative and fail to function, usually without disconnecting thepolyphase apparatus from the polyphase sources. Since these devicesoperate upon the phase relationship between the polyphase sources, atemporary shift in the phase relationship between these phase sourceswould activate this type of device giving a false indication of phasefailure.

It is, therefore, an object or" the present invention to provide a noveldevice for detecting and indicating a phase failure in polyphasesources.

Another object is to provide a device capable of detecting andindicating a phase failure in polyphase sources where a counter voltageis generated into the phase which has failed.

A different object is to provide a device capable of detecting adifference in A.-C. voltage between one phase source and another orbetween one phase source and the remaining phase sources.

Another object is to provide a phase failure detection device that isusable with polyphase sources having a variable current loading andunequal currents in the phase sources such as results when polyphase andsingle-phase current operated devices are intermittently andsimultaneously connected to a common polyphase source.

A different object is to provide a device of the present invention thatis not falsely actuated by the normal fluctuations in the voltage in thepolyphase sources.

Another object is to provide a device of the present invention that canbe used on polyphase sources which have ditferent voltages in each phasesource under normal operating conditions.

A diiferent object of the present invention is to provide a device thatfor its operation does not require the total sum or difi erence in thevoltages or currents in the polyphase sources to reach a predeterminedpercentage of a given value before it is actuated.

Other objects, advantages and features of this invention will beapparent to one skilled in the art upon a consideration of the writtenspecification, the attached claims and the annexed drawings.

In the drawings, wherein like reference characters are used throughoutto designate like parts:

FIG. 1 is a schematic illustration of a novel phase failure detectiondevice embodying this invention; and

FIG. 2 is a diagrammatic illustration of the above detection deviceschematically connected to a polyphase motor which it protects in theevent of failure of one or more of the phase sources.

Referring to FIGS. 1 and 2, a detailed description of a preferredembodiment of the present invention will be given.

The objects of the present invention are accomplished by a device havinga D.-C. circuit between each of the voltage sources of a polyphasesource and a conductor common to all of such phase sources. Means areprovided in each such ll-C. circuit to establish a D.-C. voltage thereinproportional to the A.-C. voltage in the phase source to which suchD.-C. circuit is connected. Signal means responsive to unbalanced D.-C.potentials to indicate a phase failure are connected be tween the D.-C.circuits. The signal means are connected to the respective D.-C.circuits at substantially the same normal D.-C. potential. A phasefailure will produce an unbalanced DC. voltage in such signal means toactivate same.

This arrangement makes it possible to compare the voltage in one sourceto that in one or more of the other sources to detect the failure of onephase source.

More specifically, there is shown a plurality of phase A.-C. voltagesource conductors 1d, ll and 12, together comprising a polyphase source.A like plurality of singlephase transformers if), 14 and 15 areconnected to the sources to electrically isolate the phase failuredevice from said sources and to provide A.-C. voltages to said deviceproportional to the A.-C. voltage in said sources.

The transformers are connected to said sources by primary windingsconnected to each phase of the polyphase sources. This phase connectionis accomplished by connecting the primary windings in series andconnecting the source conductors 19, ll and 12 to common junctionsbetween the windings as clearly shown in FIG. l. This is commonly knownas a delta connection.

A secondardy winding is provide in each transformer and is adapted toproduce some convenient A.-C. voltage usable in this novel device. Oneend of each secondary winding is connected to a junction and to a commonconductor 16. The conductor in will be common to each of the other endsof the secondary and also to each of the source conductors lit, 3.1 and12-.

A plurality of D.-C. circuits 1?, is and 19, to be more fully describedthereafter, is connected between the other ends of the secondarywindings and the common conductor 16.

The transformers may be found unnecessary where a four-wire, three-phaseelectrical system is available as the polyphase A.-C. voltage source andelectrical isolation of this novel device from said source is notdesired. in such instance, the neutral or fourth wire becomes the commonconductor 16 and the polyphase source conductors are connected directlyto D.-C. circuit 37, 1S and 19.

The D.-C. circuit 17, 1.8 and 19 have connected therein, in series,rectifier means 2%, 21 and 22, respectively, and potential droppingmeans 23, 2d and 25, resepctively. Fetential storing means 26, 27 and 28are connected in shunt across respective potential dropping means 23, 24and 25.

The rectifier means it 21 and 22 rectify the A.-C. voltage from thesecondary winding of transformers 3.3, 14 and 15 to produce a D.-C.voltage and may be of any type capable of rectifying A. C. voltage suchas selenium rectifiers.

The potential dropping means limit the current passing through therectifier means to a safe and desired value and may be of any type ofelectrical resistance, such as wire wound resistors. As a result of thiscurrent flow, each potential dropping means will have a certain D.-C.potential developed across it upon rectification of the A.-C. voltage.

The potential storing means provide a constant D.-C. voltage across thepotential dropping means and may be of any type to accomplish thispurpose such as electrolytic condensers.

The potential dropping means 23, 24 and 25 are provided with potentialdividing taps 29, 39 and 31, respectively. The taps may take any formsuch as a fixed or variable contact on wire wound or carbon resistances.However, it has been found preferable to use a variable contact type inthe form. of a potentiometer for the potential dropping means and toprovide a variable potential dividing tap. in this instance the fixedresistance portion of the potentiometer corresponds to t1 e potentialdropping means and the arm becomes the variable tap.

The D.-C. circuit as above described have provided means whereby a D.-C.potential is created in each such circuit proportional to the A.-C.voltage between the phase source conductor to which such D.-C. circuitis connected and the common conductor. Each of the variable taps 2-9,Ell and 31, may be adjusted to the same D.-C. potential. Of course, thislatter selected D.-C. potential ill be proportional to the A.-C. voltagebetween the phase source conductor to which the corresponding D.-C.circuit is connected and the common conductor.

A plurality of signal means 32 and 33 are provided to indicate thefailure of one of the polyphase sources. The signal means are activatedby an unbalanced D.-C. voltage applied to them. The signal means may beof any type adapted to be activated by an unbalanced D.-C. potential,such as lights, galvanorneters of preferably D.-C. voltage relays asshown in this embodiment.

The use of a D.-C. relay as a signal means allows the device to not onlydetect and indicate a phase failure but also to be adapted to provide aload control means for disconnecting the polyphase source from itscurrent consuming loads.

As shown in PEG. 1, the relays 32 and 33 are provided with electricalenergiz g windings adapted to operate the relay when subjected to anunbalanced D.-C. voltage. The energizing winding of relay 32 isconnected between taps and in D.-C. circuits l7 and 18. The energizingwinding of relay 33 is connected between taps 3i) and 31 in D.-C.circuits 1% and 19.

A D.-C. voltmeter 42 is provided in conjunction with switch means 43 sothat the ll-C. potentials at taps 29, 3d and 31 with respect to thecommon conductor 15 may be determined.

The switch means 43 may be of any convenient type for connecting thevoltmeter to the taps and has positions corresponding to taps 29, 3d and3E which positions are designated 434, 43% and 43-3.

Although the relays will give an indication of phase failure, it hasbeen found that such relays may be easily adapted to disconnect thepolyphase load from its polyphase source. For this purpose relays 32 and33 are provided with electrical switch contacts 35 and 35 actuated bythe operation of the relays.

The relay switch contacts 35 and as are connected in series with loadcontrolling relay 34 in a circuit to be more fully described hereafterand these switch contacts have a bypass switch 4i connected in shunt.

The load controlling relay 34 is provided with an energizing winding andswitch contacts 37 actuated by operation of the relay. The energizingwinding is connected between one of the secondary windings oftransformers 13, 14 or 15 with the switch contacts 35 and 36 interposedin this circuit. Thus, it can be seen that opening either one of theswitch contacts 35 and 36 would de-energize the energizing winding ofthe load controlling relay opening switch contacts 37. The switchcontacts 37 are connected to a load control means by conduits 38 and 3%.

The bypass switch 40 allows the relay 34 to be energized while the taps29, 3G and 31 in the D.-C. circuits are adjusted to normal operation orfor test purposes.

It has been found desirable to place the transformers, relays and otherportions of this novel device in a container 41 shown in FIG. 1 by adotted outline.

The novel phase failure detection device of the present invention isadjusted and operates as follows.

With normal distribution voltages present in the polyphase source, thebypass switch 46 is closed activating relay 34 and switch contacts 37.This results in the load control means connecting the load to thepolyphase source. Since the current consuming load may be polyphase,singlephase or a combination of both types of load, the currents andvoltages in the source conductors 10, 11 and 12 probably will not beidentical and most frequently are not identical. Thus, it is preferableto have all current consuming loads connected at the time of adjustmentof this novel device.

A D.-C. potential is produced in each D.-C. circuit proportional to theA.-C. voltage in the source connected thereto. The voltages across eachrelay 32 and 33 are balanced by adjusting taps 29, 3t) and 51 tosubstantially the same predetermined D.-C. voltage. When the D.-C.voltage is balanced across the relays 32 and 33, the windings of theserelays are not energized and electrical switch contacts 35 and 36 remainclosed completing the A.-C. energizing circuit of the load control relay34.

After the D.-C. voltages are adjusted, the bypass switch 40 is openedand this novel device assumes automatic phase failure control.

When a phase failure occurs, a drop in the A.-C. voltage in one or morephase conductors results. Naturally, there is a proportional drop in theD.--C. voltage in the corresponding Ill-C. circuit but not in theothers. The change in -0. voltage in one D-C. circuit causes the D.-C.voltage across one of the relays connected thereto to become unbalanced.This resultant unbalanced D.-C. voltage is proportional to thedifference in the changes in A.-C. voltages in the source conductors. Ifthe unbalance in A.-C. voltage is sufficient, as when a phase fails,there results an activating of one relay to give indication of suchphase failure and a simultaneous opening of the electrical switchcontacts actuated by that relay. Thus, phase failure detection andindication is accomplished and the load control relay is de-energized.As a result, the load control means disconnects the load from saidsource.

The D.-C. voltage at each of the taps 29, and 31 is of a value such thatnormal voltage fluctuations in the sources are insufficient to cause anunbalanced D.-C. voltage of a sufficient magnitude to activate either ofthe relays. Additionally, the value should be such that the countervoltage generated into the failed phase by certain apparatus is notsuflicient to produce a D.-C. potential sufficiently close to normal tomaintain the D.-C. balance across the relays and prevent theiractuation.

Examination of FIG. 1 shows that the voltage 6 may be easily selected ineach D.-C. circuit by means of the variable taps. This D.-C. voltage canbe set between a value approaching zero and the maximum D.-C. voltageavailable across the potential dropping means. However, the higher theD.-C. voltage obtained at the variable taps compared to the total D.-C.voltage across the potential dropping means, the greater the sensitivityof this novel device to changes in the voltages in the phase sources.

As an example, where the total D.-C. voltage across the potentialdropping means is 300 volts and the un balance in DA). voltage acrossthe relays necessary to activate them is 10 volts, a setting of the taps29, 3% or 31 at volts will require a DC. voltage change caused by phasefailure of 30 volts or more before one of the relays can be activated.

A setting of the taps at 200 volts would require a D.-C. voltage changeof only 15 volts to cause an unbalanced D.C. voltage of 10 volts. Thus,the sensitivity of the device at 200 volt settings of the taps would betwice that of 100 volt settings of the taps.

Clearly, the D.-C. voltage at which the taps are set can bepredetermined at a value such that normal fluctuations are not of asutiicient magnitude to activate the relays and the counter voltageintroduced into the phase which has failed is insufficient to preventactivation of the relays.

The D.-C. voltage setting at the taps can be determined for a givenA.-C. voltage fluctuation and counter voltage since the il-C. potentialin the D.-C. circuit is proportional to the A.-C. voltage in the sourceconductor to which that circuit is connected and the unbalanced D.-C.voltage across the relays is proportional to total D.-C. voltage acrossthe potential dropping means and the D.-C. voltage to which the taps areset.

Referring now to FIG. 2, a detailed description of a typical use of thenovel device to control a polyphase load will be given. The load isshown as a polyphase motor.

A polyphase electric motor 44 is connected through a load control meansto the polyphase source conductors 1d, 11 and T2. The load control meansis here shown as the usual motor controller relay 45 having anenergizing winding and switch contacts (l6, 47, 48 and 49 actuated byoperation of the relay. The switch contacts 46, 47 and 48 are interposedin the sources to connect same to the motor 44. The contacts 4% areauxiliary contacts and are used in a load control circuit hereafterdescribed and to bypass a starting switch St The switch 50 is of thetype which opens upon removal of the force closing same.

The phase failure device is housed in container 41 and isdiagrammatically illustrated with the controls thereon as follows. Thebypass switch 4% has a control knob 43a extending outside of thecontainer 41. The voltmeter 42 is placed so that its face is readilyvisible. The taps 2.5 359 and 31 have control knobs 2%, Ella and 31alikewise extending outside of the container 41 as does the bypass switchknob ilia.

This novel device is connected to the motor controller relay andpolyphase sources through the following load control circuit.

This control circuit has polyphase source conductors l1, l2 and 13connected to the respective parts of the novel device as shown in FIGS.1 and 2. Conduits 38 and 39 extend from container 41.

Conduit 38 is connected through auxiliary switch contacts 4-9 to sourceconductor ll). Conduit 39 is con nected through a stop switch 51,through the energizing winding of the motor controller relay 45 tosource conductor 11.

The starting switch 59 is connected between source conductor Ill andconduit 39.

Upon the novel device being adjusted for automatic phase failuredetection as has been previously discussed, the circuit through conduits33 and 39 is therefore completed by the closing of electrical switchcontacts 37. The

sucrose a motor controller relay windin is energized closing switchcontacts 46, 47, 48 and 49 so that the polyphase motor 44 is connectedto source conductors Zltl, Ill and l2.

Upon a phase failure, as heretofore described, the circuit throughconduits S3 and E9 is broken by the opening of the load control relayswitch contacts 3?. This results in tie-energizing the motor controllerrelay 45 and opening the switch contacts actuated thereby.

The starting switch is paralleled by auxiliary switch contacts 49 andtherefore may be released upon the phase failure detector assumingautomatic control.

The motor 44 can be stopped when desired by opening stopping switch 51.

The phase failure device and control circuits between the device and themotor controller relay 45 may be checked when the motor controller relayhas disconnected the motor 44 from the polyphase source. Closing starterswitch Ell will result in re-connecting the motor to the polyphasesource where the control circuit has been broken by a defective stoppingswitch and/or auxiliary contacts 4-9 upon normal voltages being presentin the source. The phase failure device is functioning properly it themotor starts.

Should the motor fail to start by the above test, the bypass switch 40is closed and the above test repeated. It the motor fails to start,there is a defective circuit condition in the control circuit. if themotor starts, some defect has arisen in the phase failure detector.

Naturally, the presence of normal operating voltages i the polyphasesource can be readily determined y use of the meter swtcli 43. The meterswitch 43 is changed through positions 434, 432 and 43-3 to determinethe presence of DC. voltages at the taps 29, 3d and 31. Should thepredetermined D.-C. voltage be present, then failure of the motor tostart under the above tests indicates a defect in the control circuitand not in the phase failure device.

Although in this embodiment the load control relay 34 is used to controlthe connection of the source to the motor by means of the motorcontroller relay, it is envisioned that the load control relay as may beprovided with switch contacts as, 47, 4-3 and 49 substituted for switchcontacts 37. In such an arrangement the starter switch 5t) would beconnected in shunt to the bypass switch 50. The stop ping switch 51would be interposed in series with relay switch contacts 35 and 35. Thisarrangement would open ate in the same manner as in the preferredembodiment.

It is evident that should the A.-C. voltages in all phase sourcesdecrease proportionally in voltage below an A.-C. voltage for safeoperation, a condition known as under voltage occurs. The 31- 3.voltages across the relays 32 and 33 would remain balanced under suchundervoltage condition and hence would not become ctivated. dividualcurrent consuming loads may be protected aga': undervoltage condition bythermal overload protect However, the present phase failure device canprotect all loads connected to the polyphase source against the undervoltage condition.

The load control relay 34 may provide such undervolb age protection forthe polyphase source. Automatic undervoltage protection for thepol phasesource provided by choosing an energizing winding for relay will doenergize at a certain voltage proportional to a predeter minedpermissible undervoltage in the source. Thus, a

source from the current consuming loads. Additionally, undervoltageprotection is provided for such loads.

The phase failure device has been herein described for use on athreephase source of electrical power. However, the device is notlimited to three-phase systems and be used with any polyphase system byproviding a D.-C. circuit for eacl polyphase source and signal meansconnected between the potential dividing taps on the potential dropp ngmeans in such D.-C. circuits.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

it will be understood that certain features and subcombinations are ofutility and may be employed without rererence to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:

l. A phase failure detector for use on an A.-C. polyphase systemcomprising, a plurality of phase voltage source conductors, a likeplurality of single phase transformers, each having a primary and asecondary winding, a primary winding connected to each of the phases ofsaid polyphase source, one end of each or" said secondary windingconnected to a common conductor, a plurality of D.-C. circuits, one ofsaid D.-C. circuits connected between tne other ends of said secondarywindings and the common conductor, said D.-C. circuits each having arectifier means and a potential dropping means connected in seriestherein, said potential dropping means having a variable potentialdividing tap, and a potential storing means shunting said potentialdropping means, a plurality of signal means responsive to D.-C. voltagedifferentials to indicate phase failure, one of said signal meansconnected between the potential dividing taps in a first and second ofsaid D.-C. circuits, another signal means connected between thepotential dividing tap in a third of said D.-C. circuits and thepotential dividing tap in the second of said D.-C. circuits, thepotential clividing taps adjusted to balance the D.-C. voltage acrosseach signal means when normal operational voltages are present in thesources and to produce an unbalanced D.-C. voltage across one signalmeans to activate same upon a predetermined unbala co in A.-C. voltagebetween any two source conductors or between one source conductor andthe remaining source conductors.

2. A phase failure detector for use on an AMI). polyphase systemcomprising, a plurality of phase voltage source conductors, a conductorcommon to each of said conductors, a plurality of D-C. ci cuits, one ofsaid D-C. circuits connected between each phase source condoctor and thecommon conductor, said D.-C. circuits each having connected in seriestherein a rectifier means nd a potential dropping means, said potentialdropping means having a variable potential dividing tap, a first relayhaving an energizing winding and electrical switch contacts actuatedthereby, said winding connected between the potential dividing taps in afirst and second of said D-C. circuits, at second relay having anenergizing winding and contacts actuated thereby, said latter windinhaving one end connected to the potential dividing tap in a third or"said D-C. circuits and the other end connected to the potential dividingtap in the second of said DJ. circuits, said first and second relaycontacts connected in series, a load controlling relay having an enrgizing win ng a plurality of contacts actuated thereby, said controlrelay contacts interposed in the enezeze 9 source conductors andconnecting said source to a load therefor, the energizing winding ofsaid load controlling relay connected between one of said sourceconductors and the common conductor and in series with the first andsecond relay contacts, the potential dividing taps adjusted to balancethe voltage across each first second relay Winding when normaloperational voltages are present in the source conductors and to producean unbalanced D.-C. voltage across one of the first and second relays tode-energize the load controlling relay thereby disconnecting said sourcefrom the load upon a predetermined unbalance in voltage between any twosource conductors or between one source conductor and the remainingsource conductors.

References Cited in the file of this patent UNITED STATES PATENTS1,314,332 Jones Aug. 26, 1919 1,888,718 Friedlander Nov. 22, 19322,242,950 Harder May 20, 1941 3,021,453 Faglie eb. 13, 1962 3,056,067Luber Sept. 25, 1962

1. A PHASE FAILURE DETECTOR FOR USE ON AN A.-C. POLYPHASE SYSTEMCOMPRISING, A PLURALITY OF PHASE VOLTAGE SOURCE CONDUCTORS, A LIKEPLURALITY OF SINGLE PHASE TRANSFORMERS, EACH HAVING A PRIMARY AND ASECONDARY WINDING, A PRIMARY WINDING CONNECTED TO EACH OF THE PHASES OFSAID POLYPHASE SOURCE, ONE END OF EACH OF SAID SECONDARY WINDINGCONNECTED TO A COMMON CONDUCTOR, A PLURALITY OF D.-C. CIRCUITS, ONE OFSAID D.-C. CIRCUITS CONNECTED BETWEEN THE OTHER ENDS OF SAID SECONDARYWINDINGS AND THE COMMON CONDUCTOR, SAID D.-C. CIRCUITS EACH HAVING ARECTIFIER MEANS AND A POTENTIAL DROPPING MEANS CONNECTED IN SERIESTHEREIN, SAID POTENTIAL DROPPING MEANS HAVING A VARIABLE POTENTIALDIVIDING TAP, AND A POTENTIAL STORING MEANS SHUNTING SAID POTENTIALDROPPING MEANS, A PLURALITY OF SIGNAL MEANS RESPONSIVE TO D.-C. VOLTAGEDIFFERENTIALS TO INDICATE PHASE FAILURE, ONE OF SAID SIGNAL MEANSCONNECTED BETWEEN THE POTENTIAL DIVIDING TAPS IN A FIRST AND SECOND OFSAID D.-C. CIRCUITS, ANOTHER SIGNAL MEANS CONNECTED BETWEEN THEPOTENTIAL DIVIDING TAP IN A THIRD OF SAID D.-C. CIRCUITS AND THEPOTENTIAL DIVIDING TAP IN THE SECOND OF SAID D.-C. CIRCUITS, THEPOTENTIAL DIVIDING TAPS ADJUSTED TO BALANCE THE D.-C. VOLTAGE ACROSSEACH SIGNAL MEANS WHEN NORMAL OPERATIONAL VOLTAGES ARE PRESENT IN THESOURCES AND TO PRODUCE AN UNBALANCED D.-C. VOLTAGE ACROSS ONE SIGNALMEANS TO ACTIVATE SAME UPON A PREDETERMINED UNBALANCE IN A A.-C. VOLTAGEBETWEEN ANY TWO SOURCE CONDUCTORS OR BETWEEN ONE SOURCE CONDUCTOR ANDTHE REMAINING SOURCE CONDUCTORS.